Department of Physics

Chameleon Star Baffles Astronomers

A pulsar with glowing cones of radiation stemming from its magnetic poles. New observations reported in Science re-open an old debate about how these spinning stars work. (Image: European Space Agency/ATG medialab)

Few astronomical objects are as baffling as pulsars, and despite nearly fifty years of study, they continue to defy theorists' best efforts.

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Pulsars — tiny spinning stars, heavier than the sun and smaller than a city — have puzzled scientists since they were discovered in 1967.

Now, new observations by an international team, including University of Vermont astrophysicist Joanna Rankin, make these bizarre stars even more puzzling.

The scientists identified a pulsar that is able to dramatically change the way in which it shines. In just a few seconds, the star can quiet its radio waves while at the same time it makes its X-ray emissions much brighter.

The research “challenges all proposed pulsar emission theories,” the team writes in the Jan. 25 edition of the journal Science and reopens a decades-old debate about how these stars work.

Unexpected X-rays

Like the universe’s most powerful lighthouses, pulsars shine beams of radio waves and other radiation for trillions of miles. As these highly magnetized neutron stars rapidly rotate, a pair of beams sweeps by, appearing as flashes or pulses in telescopes on Earth.

Using a satellite X-ray telescope, coordinated with two radio telescopes on the ground, the team observed a pulsar that was previously known to flip on and off every few hours between strong (or “bright”) radio emissions and weak (or “quiet”) radio emissions.

Monitoring simultaneously in X-rays and radio waves, the team revealed that this pulsar exhibits the same behaviour, but in reverse, when observed at X-ray wavelengths.

This is the first time that a switching X-ray emission has been detected from a pulsar.

Flipping between these two extreme states — one dominated by X-ray pulses, the other by a highly organized pattern of radio pulses — “was very surprising,” says Rankin.

“As well as brightening in the X-rays we discovered that the X-ray emission also shows pulses, something not seen when the radio emission is bright,” said Rankin, who spearheaded the radio observations. “This was completely unexpected.”

No current model of pulsars is able to explain this switching behavior. All theories to date suggest that X-ray emissions would follow radio emissions. Instead, the new observations show the opposite. “The basic physics of a pulsar have never been solved,” Rankin says.

Looking for the switch

The research was conceived by a small team then working at the University of Amsterdam, including UVM’s Rankin, who has studied this pulsar, known as PSR B0943+10, for more than a decade; Wim Hermsen from SRON, the Netherlands Institute for Space Research in Utrecht, and the lead author on the new paper; Ben Stappers from the University of Manchester, UK; and Geoff Wright from Sussex University, UK.

These researchers were joined by colleagues from institutions around the world to conduct simultaneous observations with the European Space Agency’s X-ray satellite, XMM-Newton, and two radio telescopes, the Giant Meter Wave Telescope (GMRT) in India and the Low Frequency Array (LOFAR) in the Netherlands, to reveal this pulsar’s so-far unique behavior.

“There is a general agreement about the origin of the radio emission from pulsars: it is caused by highly energetic electrons, positrons and ions moving along the field lines of the pulsar's magnetic field,” explains Wim Hermsen.

“How exactly the particles are stripped off the neutron star's surface and accelerated to such high energy, however, is still largely unclear,” he adds.

By studying the emission from the pulsar at different wavelengths, the team’s study had been designed to discover which of various possible physical processes take place in the vicinity of the magnetic poles of pulsars.

Instead of narrowing down the possible mechanisms suggested by theory, however, the results of the team’s observing campaign challenge all existing models for pulsar emission. Few astronomical objects are as baffling as pulsars, and despite nearly fifty years of study, they continue to defy theorists’ best efforts.

Of the more than 2,000 pulsars discovered to date, a number of them have erratic behavior, with emissions that can become weak or disappear in a matter of seconds but then suddenly return minutes or hours later.

B0943+10 is one of these erratic stars. Discovered at Pushchino Radio Astronomical Observatory near Moscow, “this star has two very different personalities,” that were uncovered by Svetlana Suleymanova in the 1980s, says Rankin.

“But we’re still in the dark about what causes this, and other pulsars, to switch modes,” Rankin says. “We just don’t know.”

“But the fact that the pulsar keeps memory of its previous state and goes back to it,” says Hermsen, “suggests that it must be something fundamental."

Recent studies indicate that the switch between “radio-bright” and “radio-quiet” states is correlated to the pulsar's dynamics. As pulsars rotate, their spinning period slows down gradually, and in some cases the slow-down process has been observed to accelerate and slow down again, in conjunction with the pulsar switching between bright and quiet states.

This correlation between a pulsar’s rotation and its emission has led astronomers to wonder about a connection between the star’s surface and the much-larger surrounding magnetosphere, which may extend up for 30,000 miles.

These new observations “strongly suggest that a temporary ‘hotspot’ appears close to the pulsar’s magnetic pole which switches on and off with the change of state,” says Geoff Wright, one of the team’s astronomers from the University of Sussex.

But the new results also suggest that something in the whole magnetosphere is changing suddenly and not just at the poles or other hotspots. “Something is happening globally,” Rankin says, across the whole star.

In order for the radio emission to vary so radically on the short timescales observed, the pulsar's global environment must undergo a very rapid – and reversible – transformation.

“If that is true, it means the entire magnetosphere is alive and connected in very important ways,” Rankin says, allowing a change in the pulsar’s basic mode of shining in about one second, less time than it takes it to spin once on its axis.

“Since the switch between a pulsar's bright and quiet states links phenomena that occur on local and global scales, a thorough understanding of this process could clarify several aspects of pulsar physics,” says Hermsen. “Unfortunately, we have not yet been able to explain it.”

No model works

The team planned to search for the same pattern in X-rays that has been observed in radio waves – to investigate what causes this switching behavior. They chose as their subject PSR B0943+10, a pulsar that is well known for its switching behavior at radio wavelengths and for its X-ray emission, which is brighter than might be expected for its age.

“Young pulsars shine brightly in X-rays because the surface of the neutron star is still very hot. But PSR B0943+10 is five million years old, which is relatively old for a pulsar: the neutron star's surface has cooled down by then,” explains Hermsen.

Astronomers know of only a handful of old pulsars that shine in X-rays and believe that this emission comes from the magnetic poles – the sites on the neutron star's surface where the acceleration of charged particles is triggered. “We think that, from the polar caps, accelerated particles either move outwards to the magnetosphere, where they produce radio emission, or inwards, bombarding the polar caps and creating X-ray-emitting hot-spots,” Hermsen adds.

There are two main models that describe these processes, depending on whether the electric and magnetic fields at play allow charged particles to escape freely from the neutron star's surface. In both cases, it has been argued that the emission of X-rays follows that of radio waves.

Monitoring the pulsar in X-rays and radio waves at the same time, the astronomers hoped to be able to discern between the two models.

“The X-ray emission of pulsar PSR B0943+10 beautifully mirrors the switches that are seen at radio wavelengths but, to our surprise, the correlation between these two emissions appears to be inverse: when the source is at its brightest in radio waves, it reaches its faintest in X-rays, and vice versa,” says Hermsen.

The new data also show that the source pulsates in X-rays only during the X-ray-bright phase – which corresponds to the quiet state at radio wavelengths. During this phase, the X-ray emission appears to be the sum of two components: a pulsating component consisting of thermal X-rays, which is seen to switch off during the X-ray-quiet phase, and a persistent one consisting of non-thermal X-rays.

Neither of the leading models for pulsar emission predicts such behavior.

In the second half of 2013, the team plans to repeat the same study for another pulsar, PSR B1822-09, which exhibits similar radio emission properties but with a different geometry.

In the meantime, these observations will keep theoretical astrophysicists busy investigating possible physical mechanisms that could cause the sudden and drastic changes to the pulsar's entire magnetosphere and result in such a curious flip in how they shine.